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Goal: An overview of what you can observe in nearby galaxies at millimeter wavelengths. Discuss typical intensities from several perspectives. Specific Topics: o What you see studying galaxies at mm wavelengths. o Intensities at extragalactic distances: clouds, galaxies, chunks of galaxies. o Millimeter continuum: origin and use (very briefly). Apologies in advance (but not really): I’m going to be very “CO-centric” (though not “CO- exclusive”). The reason, which I hope you’ll appreciate is that CO - by far the brightest mm line - is already very sensitivity limited at extragalactic distances. Observing Nearby Galaxies at mm s

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Starbursts and Associations of GMCs Resolution and sensitivity matched to a single GMC are largely limited to the Local Group (especially for a single dish telescope). Shen & Lo ‘95 (M82, left), Roslowsky & Blitz (M64, right); Wilson+ (Antennae, bottom) Starbursts show GMC-like properties (or more extreme) over scales of few hundred pc to kpc. No longer analogs to collections of MW GMCs. Aalto+ ‘99: collections of GMCs in M51 On various scales, most mapping of other galaxies relies on getting several clouds per beam.

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Well Beyond CO R. Genzel (1991) The low rotational transitions of 12 CO are the best available simple tracer of distribution of H 2 mass. More sophisticated mm spectroscopy can reveal physical conditions (density, temperature) and refine estimates of H 2.

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But CO is Still Your Benchmark / Starting Point Sutton+ ‘85: 1mm line survey of Orion A Pointing towards the bright part of a nearby GMC: 12 CO J=2-1 : T peak ~ 110 K Next brightest line: 13 CO J=2-1 : T peak ~ 35 K Other lines down by larger factors. … The same thing is true in galaxies: o 12 CO J=2-1 (230 GHz) and J=1-0 (115 GHz) are the brightest lines. o Typically this is followed by the same transitions in 13 CO (lower  ; intensity down by a factor of ~6 in the Milky Way). o Then HCN, HCO+, CN, HNC, CS, C 18 O, etc. o Intensities for these species seldom much higher than 1/10 CO.

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Looking at Other Galaxies: Numbers For the next few slides we’ll look at other the intensity of other galaxies as observed with the 30m. We’ll do this in three ways: o In terms of individual molecular clouds … o In terms of integrated molecular mass (luminosity) … o In terms of surface density … We’ll focus on the 12 CO J=1-0 transition. As we’ve just discussed, this (and the corresponding J=2-1 transition) is the brightest available transition by a factor of several.

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Looking at Other Galaxies: Units UnitWhat Does It Measure?Notes K brightness temperature (a way to phrase specific intensity) be careful when switching to and from Jy beam -1 (factor of 2 )! K km/s integrated intensity brightness temp. integrated over velocity. with X CO yields a surface density. usually quoted as an average over a telescope beam. K km/s arcsec 2 flux brightness temp. integrated over velocity and angular area. independent of telescope beam. K km/s pc 2 luminosity brightness temp. integrated over velocity and physical area with X CO yields an integrated molecular mass.

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Looking at Other Galaxies: Conversion Factors X CO : assume a linear conversion between CO and H 2 to hold on large scales. This “X-factor” is the conversion. Milky Way value (  rays, dust emission, virial mass): X CO =  cm -2 (K km s -1 ) -1 But be careful! X CO is an approximation. It does not hold perfectly within Galactic clouds and different values are used in starburst and dwarf galaxies. Using X CO and the units just described, we can then convert: o Integrated Intensity [K km/s] to Surface Density [M sun pc -2 ] o Luminosity [K km/s pc 2 ] into Molecular Mass [M sun ] N.B., this is only H; apply another factor of ~ 1.36 to include He.

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Looking at Other Galaxies: GMC Perspective How bright are giant molecular clouds in other galaxies? What is the line-average intensity (in K) you expect pointing the 30m at a GMC in a nearby galaxy? 1. Luminosity of a GMCK km s -1 pc 2 2. Line width of a GMCkm s Size of a cloud (or the beam)pc 2 Line-average intensityK

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Looking at Other Galaxies: GMC Perspective Some things to take away: o Individual CO-emitting structures are faint at extragalactic distances. o Individual clouds only resolved inside the Local Group by the 30m. o Individual low mass clouds only detectable inside Local Group. o Individual high mass clouds detectable with effort in nearest other groups. o Observing distant galaxies requires averaging many clouds inside a beam (that’s okay, beam has large spatial area too). We’ve used 12 CO as our example, scale to get other lines.

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Looking at Other Galaxies: Galaxy Perspective How bright are whole galaxies in CO? Spatial size of the 30m beam gets very big at extragalactic distances. It is easy to find yourself with most of a galaxy inside a beam. In this case, the relevant thing is the CO luminosity of the galaxy, L CO. How to make a good guess at L CO ? o We’ve already seen that many galaxy properties are strongly covariant with one another. This include CO luminosity. o Both stellar luminosity and infrared luminosity (or some other tracer of star formation rate) are good places to start.

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Looking at Other Galaxies: Galaxy Perspective For actively star-forming galaxies, CO is clearly related to star formation (above traced by IR, but this works for other tracers, too). Details vary a bit: o In normal disks, a fixed CO-to-IR ratio is probably pretty much OK. o Very vigorous star-formers have less CO per IR. o Quiescent and low-metal galaxies may also have less CO per IR. Gao & Solomon ‘04 Star Formation Rate [from IR] H 2 Mass from CO

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Looking at Other Galaxies: Galaxy Perspective Some things to take away: o When all CO emission from a galaxy is in the beam, the 30m can detect galaxies out to a large distance, though the time investment is still not trivial. o We have ignored metallicity effects, enhancement due to intereactions. In reality, low mass galaxies are hard to see at all and mergers can be very bright. o Unlike GMCs, it isn’t always straightforward to get all of the galaxy inside the beam. We’ll talk a bit about the intermediate case next… We’ve used 12 CO as our example, scale to get other lines.

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Looking at Other Galaxies: Surface Density Often, especially when mapping, you are in the intermediate regime: Many GMCs per beam, but not a sizable piece of the galaxy. Relevant quantity here is H 2 surface density or CO surface brightness. We’ve talk about how we might estimate the CO luminosity of a galaxy, but how is this luminosity distributed? HERA (30m) maps of some normal spiral galaxies at ~10 Mpc. The spatial resolution is a kpc and the mass in each resolution element is more than 10 6 Msun, implying collections of GMCs.

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Looking at Other Galaxies: Surface Density A typical CO scale length in a massive star forming galaxy (like the Milky Way) Size of a typical Milky Way Giant Molecular Cloud We’re in this regime from just beyond the Local Group (can see clouds in SMC, LMC, M33, M31) to several 10s of Mpc (past which even big galaxies are essentially point sources).

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Looking at Other Galaxies: Surface Density Some things to take away: o Azimuthally averaged, CO emission looks pretty similar to stars (there are important differences, but this is a good place to start): an exponential decline with a scale length ~0.2 to 0.25 times the optical radius. o The exponential decline is a mix of filling factor (e.g., arms vs empty space) and decline in the peak integrated intensity (arms get fainter). o The local star formation rate or IR surface brightness are reasonable ways to guess at the surface brightness of CO on fairly large (~ kpc) scales. We’ve used 12 CO as our example, scale to get other lines or use the well- established HCN-IR (or an analogous HCO+-IR) relations.

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Millimeter Continuum Physical information from thermal free free emission: o Estimate the rate of ionizing photons hitting the region producing the emission. o Emission measure  number of recombinations, so… (Note the unit change on L ff !) See Condon ‘92 … why is this interesting? Ionizing photons are produced only by young stars, making this an effective, easily interpreted tracer of recent star formation (in practice this is slightly easier at longer wavelength than in the millimeter, e.g. K band).

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Wrap Up 1.Millimeter spectroscopy of other galaxies: Best available tool to study distribution of H 2. CO strongest line by several factors (n crit, abundance). Weaker lines give physical conditions (or fraction of dense, excited gas). 2.Sensitivity calculations / feasibility estimates: through the lens of GMCs. through the lens of H 2 surface densities. for whole galaxies. Optically thinner, high excitation, high n crit lines: down by factor … 3.Continuum: mixture of free-free and dust. Weak, often filtered out by design. Carries info about ionizing radiation, dust temperature and dust mass (AW).